The present invention relates generally to method of fabricating fiber Bragg gratings, and in particular to method of fabricating multiple superimposed fiber Bragg gratings.
Fiber Bragg grating is now a key device in the established and emerging fields of optical communication and optical fiber sensing. There are basically two methods to photo-induce gratings in photosensitive optical fiber wave-guides, internal writing method and external writing method. The internal writing method was first described by K. O. Hill et al. and was disclosed in U.S. Pat. No. 4,474,427. In this method, coherent light having a wavelength in the visible region is launched into the core of a Ge-doped fiber from one end of the fiber. The light is reflected from the other end of the fiber. The forward propagating light and the backward propagating light interfere with each other to produce a standing wave with a period corresponding to half the wavelength of the writing light. By a photosensitive effect in the fiber, a refractive index grating with the period of the standing wave is written into the core of the fiber. The main drawback of this internal writing method is that only gratings with a period similar to that of a one-half the wavelength of the writing light can be made. The application of those fiber gratings written by internal writing method is substantially limited due to this drawback, especially in fiber communication areas.
Three general approaches of external writing method have been developed to overcome the drawbacks of internal writing method and actually make the application of fiber Bragg gratings in optical fiber communication areas possible. The first approach of external writing method was demonstrated by Glen et al. and was disclosed in U.S. Pat. No. 4,807,950. In this approach, an interferometer is used to split an incoming UV light into two beams that were subsequently recombined to form an interference pattern that side exposes a photosensitive fiber, inducing a permanent refractive index modulation in the core. As the Bragg grating period (which is identical to the period of the interference fringe pattern) of a fiber Bragg grating written in the core depends on both the irradiation wavelength and the half angle between the intersecting UV beams, theoretically Bragg gratings at any desired wavelength can be inscribed. The disadvantages of this approach include a susceptibility to mechanical vibration, relative complexity of the system and requirement for a UV laser source with good spatial and temporal coherence.
U.S Pat. No. 5,104,209 discloses a point-by-point approach of external writing method for fabricating Bragg gratings by inducing a change in the index of refraction corresponding to a grating plane one step at a time along the core of the fiber. The main advantage of this approach is its flexibility to alter the Bragg grating parameters. However, the point-by-point approach is a tedious process requiring a relatively long process time. Errors in the grating spacing due to thermal and/or mechanical vibration can occur. This limits the grating to a short length.
One of the most effective and mature approaches of external writing method for inscribing Bragg gratings in photosensitive fiber is the phase-mask approach. U.S. Pat. No. 5,367,588 discloses a phase-mask approach that employs a phase-mask (a diffractive optical element) to spatially modulate the UV writing beam. Phase-mask may be formed either holographically or by electron-beam lithography. The phase-mask is created as a one dimensional periodic surface-relief pattern. The profile of the periodic surface-relief grating is selected such that when a UV beam is incident on the phase-mask, the zero-order diffracted beam is suppressed to less than a few percent of the transmitted power. In addition, each of the diffracted plus and minus first orders is maximized to typically contain more than 35% of the transmitted power. The interference of the diffracted plus and minus first order beams produces a near-field fringe pattern with a period that is one-half of that of the phase-mask. The fringe pattern photo-imprints a refractive index modulation in the core of a photosensitive optical fiber that is placed in contact with or in close proximity to the phase-mask. Since its original demonstration in 1993, the phase-mask approach has been developed to a stage where the inscription of a nearly 100% reflective grating is now routine. U.S. Pat. No. 5,903,689 discloses a phase-mask-based method for spatially controlling the period and amplitude when inscribing a fiber Bragg grating in a photosensitive fiber.
As only one optical element is used to provide a robust and inherently stable method for producing fiber Bragg grating, the phase-mask approach substantially reduces the complexity and the cost of a fiber grating fabrication system. Since the fiber is usually disposed directly behind the phase-mask in the near field of the diffracted UV beams, sensitivity to mechanical vibration and therefore stability problems are minimized. Also, low temporal coherence does not affect the writing capacity as compared to the interferometric approach. However, the spatial coherence still plays an important role in the fabrication of Bragg gratings.
Multiple superimposed fiber Bragg gratings have been of great interest as a device in optical communications, lasers and sensor systems because multiple Bragg gratings at the same location basically perform a comb function that is ideal for manipulating, e.g. multiplexing and de-multiplexing, signals with different wavelengths. Writing all gratings at the same location of a fiber is well suited for optical integrated technology, where the physical size of a device is always a concern. Another advantage is the simplicity and cost-effectiveness for the athermal package structure that is one of the key technologies in fiber Bragg grating area. One general package can compensate the temperature induced wavelength shifts at the same time in all superimposed gratings at the same location.
Chirped Bragg gratings are highly valued in dispersion compensation applications of high-speed optical communication system. However, meaningful multi-channel dispersion compensation needs to cascade a plurality of single fiber Bragg gratings (each of them has a unpacked length of more than 10 cm) together that will result in large physical size and more complicated structure. Superimposing several chirped Bragg gratings at a same location can effectively reduce the physical size of such devices and can also substantially simplify the structure.
Multiple superimposed fiber Bragg gratings can also be used for material detection where the multiple Bragg lines can be designed to match the signature frequencies of a given material.
U.S. Pat. No. 5,627,927 discloses the use of two or more Bragg gratings superimposed at a same location of an optical fiber for sensing environment effect such as strain and temperature. However, this prior art reference does not specifically teach how the multiple superimposed Bragg gratings used are inscribed in the core of a photosensitive fiber.
U.S. Pat. No. 6,275,511 teaches two methods to create multiple superimposed Bragg gratings (column 4, lines 59-66). Both methods are on the basis of the phase-mask approach. The first method photo-induces multiple superimposed Bragg gratings by overwriting each of the fiber Bragg gratings with a corresponding phase-mask in an optical fiber. This method uses as many phase-masks as the number of gratings to be written in an optical fiber. Due to the changing of masks between each two consecutive writings, the configuration of the writing system can easily be altered and the repeated calibrations make the writing procedure a tedious process. The second method photo-induces multiple superimposed Bragg gratings by overwriting all of the fiber Bragg gratings with a single specially designed phase-mask that can generate interference patterns for all fiber Bragg gratings at the same time. This method actually transfers a part of the difficulty of writing multiple superimposed Bragg gratings into the difficulty of designing and manufacturing a more complicated phase mask. This method is not flexible or cost-effective because for each possible combination (number of gratings, different wavelengths required, wavelength intervals required) of fiber Bragg gratings to be written, a specially designed phase-mask is needed.
In optical communication applications, especially DWDM, OADM et al., the channel spacing and central wavelength accuracy for the multiple channels should generally obey the ITU-T grid. For a single fiber Bragg grating, the Bragg wavelength can be tuned, e.g. by altering the strain of the fiber, to a required valve during packaging process. If a device is composed by cascading several single fiber Bragg gratings with different wavelengths together, the wavelength interval of between two single Bragg gratings can be tuned by tuning their wavelength separately. Unfortunately, the wavelengths of multiple superimposed fiber Bragg gratings can generally only be linearly shifted together, e.g. by altering the strain of the fiber, during packaging process and the wavelength interval between two fiber Bragg gratings superimposed is generally not adjustable during packaging process.
Therefore, it is greatly desired that the wavelength interval of two superimposed fiber Bragg gratings can be finely tuned to a required value during writing procedure. It is also greatly desired that the wavelength intervals of three or more superimposed fiber Bragg gratings can be optimally tuned to a required wavelength grid, e.g. an ITU (International Telecommunication Union) wavelength plan, with an acceptable tolerance during writing procedure. It is also desired that the long-term stability of gratings can be improved by uniform UV exposure.
In view of the above, it would be an advance in the art to provide a method of fabricating multiple superimposed fiber Bragg gratings which uses only a single common phase-mask.
It would be a specially welcome advance to provide a method of fabricating multiple superimposed fiber Bragg gratings where the wavelength interval of two superimposed fiber Bragg gratings can be finely tuned to a required value and the wavelength intervals of three or more superimposed fiber Bragg gratings can be optimally tuned to a required wavelength grid, either even or uneven.
It is a primary object of the present invention to provide a method of fabricating multiple superimposed fiber Bragg gratings in the core of a portion of a photosensitive fiber where only one common phase-mask is used. The superimposed multiple fiber Bragg gratings are inscribed in the portion of the photosensitive fiber by applying different predetermined longitudinal stresses to the portion for each corresponding writing.
It is a further object of the present invention to provide a method of fabricating multiple superimposed fiber Bragg gratings where different predetermined longitudinal stresses are applied to a portion of a photosensitive fiber by stretching or relaxing the portion along its longitudinal axis bi-directionally and simultaneously such that the centers of all fiber Bragg gratings imprinted in the portion are substantially superimposed at the center of the portion of the photosensitive fiber.
It is yet another object of the present invention to provide a method of fabricating multiple superimposed fiber Bragg gratings with a period of either even or uneven where the wavelength interval of two superimposed fiber Bragg gratings can be finely tuned to a required value and the wavelength intervals of three or more superimposed fiber Bragg gratings can be optimally tuned to a required wavelength grid, either even or uneven, by changing the effective refractive index of a portion of a photosensitive fiber covering the multiple fiber Bragg gratings inscribed.
It is yet another object of the present invention provide a method of fabricating multiple superimposed fiber Bragg gratings where the effective refractive index is changed by applying, for a predetermined period of time, a substantially uniform UV beam directly on the portion of a photosensitive fiber covering the multiple fiber Bragg gratings inscribed.
It is yet another object of the present invention provide a method of fabricating multiple superimposed fiber Bragg gratings where the predetermined period of time for applying a substantially uniform UV beam directly on the portion of a photosensitive fiber is decided by an optimal method, e.g. minimum mean square error method.
As there is only one common phase-mask is required in the method of present invention for fabricating multiple superimposed fiber Bragg gratings at a same location of a photosensitive fiber, most existing phase-mask based fabricating systems for inscribing single fiber Bragg grating can be used, except for adding means for illuminating substantially uniform UV light beam and means for stretching the portion along its longitudinal axis bi-directionally and simultaneously. Most of commercially available phase-masks can be used in the method of the present invention. The method of the present invention is not only cost effective but also suitable for mass production.
These and numerous other objects and advantages of the present invention will become apparent upon reading the detailed description.
In accordance with the present invention, there is provided a method of fabricating a first Bragg grating and a second Bragg grating in a portion of a photosensitive fiber. The first and the second Bragg gratings are overlapped and have a required Bragg wavelength interval S between each other.
The method has steps of disposing an optical phase-mask adjacent to the portion of the photosensitive fiber; subjecting the portion to a first predetermined longitudinal stress; applying a collimating UV light beam through the phase-mask to create the first Bragg grating in the portion; subjecting the portion to a second predetermined longitudinal stress; and applying the collimating UV light beam through the phase-mask to create the second Bragg grating in the portion. The first and the second predetermined stresses are selected to produce a tentative Bragg wavelength interval Sxe2x80x2 between the first and the second Bragg gratings such that the tentative Bragg wavelength interval Sxe2x80x2 is slightly smaller than the required Bragg wavelength interval S. The method also has step of tuning the tentative Bragg wavelength interval Sxe2x80x2 substantially to the required Bragg wavelength interval S by changing the effective refractive index of the portion covering the first and second fiber Bragg gratings.
The step of tuning the tentative Bragg wavelength interval Sxe2x80x2, substantially to the required Bragg wavelength interval S can be performed by applying a substantially uniform UV light beam directly on the portion covering both the first and the second Bragg gratings for a predetermined period of time. It should be noted that other methods, either chemical, physical or mechanical, which can change the effective refractive index of a portion of a photosensitive fiber can be used in present invention for tuning the wavelength interval between the two fiber Bragg gratings.
The steps of subjecting the portion to a first predetermined longitudinal stress and subjecting the portion to a second predetermined longitudinal stress are performed by stretching or relaxing the portion bi-directionally and simultaneously such that the center of the portion is kept substantially fixed with respect to the phase mask. The phase mask can be an even period phase mask, a non-even period phase mask or their equivalents.
The collimating UV light beam is provided by a UV source selected from a group consisting of excimer laser, frequency-doubled dye laser, frequency-doubled parametric oscillator, argon ion laser and copper vapor laser. The collimating UV light beam and the substantially uniform UV light beam can be provided by a same UV source.
The portion of the photosensitive fiber is coupled to a source and an optical spectral analyzer (OSA) for monitoring. The source is selected from a group consisting of white source, e.g. a broadband LED/SLED, and wavelength tunable source, e.g. a tunable diode laser.
The method can further have step of tuning the Bragg wavelength of the first Bragg grating by changing the effective refractive index of the portion before creating the second Bragg grating, e.g. by applying a substantially uniform UV light beam directly on the portion.
It is apparent to those skilled in the art that the method should not be considered to be limited in inscribing multiple superimposed fiber Bragg gratings with only two Bragg gratings. This method is well suited for inscribing multiple superimposed fiber Bragg gratings with two or more Bragg gratings. The first and the second fiber Bragg gratings should be considered as any two of all fiber Bragg gratings superimposed at a same location/portion of a photosensitive fiber. The tentative Bragg wavelength interval Sxe2x80x2 and the required Bragg wavelength interval S between the first and the second fiber Bragg gratings should be considered as the tentative Bragg wavelength interval and the required Bragg wavelength interval between any two of all fiber Bragg gratings superimposed at a same location/portion of a photosensitive fiber.
In accordance with the present invention, there is further provided a method of fabricating a first Bragg grating, a second Bragg grating and a third Bragg grating in a portion of a photosensitive fiber. The first, the second and the third Bragg gratings are overlapped and have a first required Bragg wavelength interval S1 between the first and the second Bragg gratings and a second required Bragg wavelength interval S2 between the second and the third Bragg gratings.
The method has steps of disposing an optical phase mask adjacent to the portion; subjecting the portion to a first predetermined longitudinal stress; applying a collimating UV light beam through the phase mask to create the first Bragg grating in the portion; subjecting the portion to a second predetermined longitudinal stress; applying a collimating UV light beam through the phase mask to create the second Bragg grating in the portion; subjecting the portion to a third predetermined longitudinal stress; applying the collimating UV light beam through the phase mask to create the third Bragg grating in the portion.
The first, the second and the third predetermined stresses are selected to produce a first tentative Bragg wavelength interval S1xe2x80x2 between the first and the second Bragg gratings and a second tentative Bragg wavelength interval S2xe2x80x2 between the second and the third Bragg gratings such that the first tentative Bragg wavelength interval S1xe2x80x2 and the second tentative Bragg wavelength interval S2xe2x80x2 are slightly smaller than the first required Bragg wavelength interval S1 and the second required Bragg wavelength interval S2 respectively.
The method also has step of tuning the first and the second tentative Bragg wavelength intervals S1xe2x80x2, S2xe2x80x2 optimally to the first and the second required Bragg wavelength intervals S1, S2 by changing the effective refractive index of the portion. The step of tuning the first and the second tentative Bragg wavelength intervals S1xe2x80x2, S2xe2x80x2 optimally to the first and the second required Bragg wavelength intervals S1, S2 can be performed by applying a substantially uniform UV light beam directly on the portion covering the first, the second and the third Bragg gratings for a predetermined period of time. It should be noted that other methods, either chemical, physical or mechanical, which can change the effective refractive index of a portion of a photosensitive fiber can be used in present invention for optimally tuning the first and the second tentative Bragg wavelength intervals to the first and the second required Bragg wavelength intervals.
The predetermined period of time can be decided by an optimal method, e.g. minimum mean square error method. It is also apparent to those skilled in the art that other optimal method can be used in the method of the present invention to decide the predetermined period of time for tuning on the basis of the power of the UV source and the photosensitive property of the fiber.
The steps of subjecting the portion to a first predetermined longitudinal stress, subjecting the portion to a second predetermined longitudinal stress and subjecting the portion to a third predetermined longitudinal stress are performed by stretching or relaxing the portion of the photosensitive fiber bi-directionally and simultaneously such that the center of the portion is kept substantially fixed with respect to the phase mask.
The phase mask can an even period phase mask, a non-even period phase mask or their equivalents.
The collimating UV light beam is provided by a UV source selected from a group consisting of excimer laser, frequency-doubled dye laser, frequency-doubled parametric oscillator, argon ion laser and copper vapor laser. The collimating UV light beam and the substantially uniform UV light beam can be provided by a same UV source.
The portion of the photosensitive fiber is coupled to a source and an optical spectral analyzer (OSA) for monitoring. The source is selected from a group consisting of white source, e.g. a broadband LED/SLED, and wavelength tunable source, e.g. a tunable diode laser.
The method can further have step of tuning the Bragg wavelength of the first Bragg grating by changing the effective refractive index of the portion before writing the second Bragg grating, e.g. by applying a substantially uniform UV light beam directly on the portion. The method can also further have step of tuning the Bragg wavelength of the first Bragg grating, the Bragg wavelength of the second Bragg grating and the Bragg wavelength difference between the first and the second Bragg gratings by change the effective refractive index of the portion, e.g. by applying a substantially uniform UV light beam directly on the portion, before writing the third Bragg grating. It is apparent to those skilled in the art that other methods, either chemical, physical or mechanical, which can change the effective refractive index of a portion of a photosensitive fiber can be used in the method of the present invention.
It is apparent to those skilled in the art that the method should not be considered to be limited in inscribing superimposed multiple fiber Bragg gratings with only three Bragg gratings. This method is well suited for inscribing superimposed multiple fiber Bragg gratings with three or more Bragg gratings. The optimal method used in the present invention to decide the predetermined period of time for optimally tuning the tentative Bragg wavelength intervals to the required Bragg wavelength intervals, e.g. an ITU wavelength plan/grid, is well suited for fabricating superimposed multiple fiber Bragg gratings with three or more fiber Bragg gratings.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The figures and the detailed description will more particularly exemplify these embodiments.